This invention relates to a polishing head for use with a semiconductor wafer polishing machine. Semiconductor wafer polishing machines are well known in the art, and are conventionally used to polish and planarize semiconductor wafers, which may include one or more photolithographic layers. Such polishing machines typically include one or more polishing heads, each of which supports a respective semiconductor wafer and positions the wafer adjacent a polishing pad. The polishing head is moved relative to the polishing pad, and a suitable polishing slurry is introduced between the wafer and the pad. U.S. patent application Ser. No. 8/287,658, filed Aug. 9, 1994, and assigned to the assignee of the present invention, discloses one such polishing machine that utilizes a belt-type polishing pad. Other polishing machines use rotary polishing pads, and are disclosed for example in U.S. Pat. Nos. 5,329,732 and 5,329,734.
Typically, a polishing head includes a central wafer carrier that is surrounded by a wafer retainer. The wafer carrier and the retainer cooperate to form a wafer-receiving pocket that prevents the wafer from moving laterally with respect to the polishing head during the polishing operation. It has been proposed to mount both the wafer carrier and the wafer retainer for relative movement with respect to the remainder of the polishing head and to bias the carrier and the retainer outwardly, toward the polishing pad. When this is done, both the retainer and the carrier are allowed to float to a limited extent with respect to the polishing head during the polishing operation.
Conventional wafer polishing operations typically include means for dressing the polishing pad, as for example by biasing a roughened element such as a ceramic element or a diamond-coated element into contact with the polishing pad in order to provide a consistent and effective polishing surface to the pad. One approach of the prior art is to apply the pad dressing element to the pad intermittently, between wafer polishing operations. This approach slows the throughput of a wafer polishing machine excessively. Another approach of the prior art is to devote one of the polishing heads of a multi-head polishing machine to pad dressing. With this approach a pad dressing element is mounted to a polishing head in substitution for a semiconductor wafer. While this approach allows pad dressing to be carried on during the wafer polishing operation, it also reduces throughput of the wafer polishing machine. This is because one of the wafer polishing heads is used for pad dressing, and not for wafer polishing.
It would be highly advantageous if it were possible to accomplish required pad dressing activities while increasing throughput of a wafer polishing machine.